The invention is broadly applicable to forming a seal with, or between, glass, metal and ceramic components. It is particularly applicable to producing envelopes for cathode ray tubes. However, it also finds application in other types of display devices and in lighting products.
It is customary in producing cathode ray tube envelopes to press funnel and faceplate components separately. These components are then joined with a fusion seal employing a mid-temperature sealing glass frit.
A successful sealing glass flit must provide adequate flow at the sealing temperature to allow the glass to wet the substrate and form a seal therewith. It must also provide a coefficient of thermal expansion (CTE) that is compatible with the CTE of the substrate.
Where a sealed device, such as a cathode ray tube, must be baked out, that is reheated under vacuum, there is a further requirement. The seal must exhibit a high viscosity at the temperature of the bakeout to avoid any visco-elastic distortion of the seal. This requirement is seemingly contradictory to the requirement of adequate flow.
These contradictory requirements are currently met by devitrifying glass frits based on PbO--ZnO--B.sub.2 O.sub.3 compositions. The key to their successful performance is the addition of a small amount of a crystallization-inducing catalyst as a powdered addition to the ball-milled base frit. The crystallization catalyst serves as a source of heterogeneous nucleation. After a brief period of flow at sealing temperatures, the frit will then undergo crystallization and become rigid. Accompanying this crystallization is a rapid change in frit viscosity, with the softening point of the fit increasing from about 350.degree. C. to approximately 575.degree. C.
The lead-zinc-borate glasses have proven very successful for the purpose. However, there has been a continuing search for a sealing material having even better sealing characteristics. Recently, this search has been accelerated by the desire to eliminate lead as a glass component.
The materials search led to development of tin-zinc-phosphate glasses as described in U.S. Pat. No. 5,246,890 (Aitken et al.) and U.S. Pat. No. 5,281,560 (Francis et al.). The glasses described in these patents are lead-free, and provide somewhat lower sealing temperatures in the range of 400-450.degree. C. with hold times no more than an hour.
The Aitken et al. glasses are of particular interest for use in producing seals in cathode ray tube envelopes because of their relatively low tin oxide contents. In addition to being lead-free, these glasses have compositions containing 25-50 mole % P.sub.2 O.sub.5 and SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range of 1:1 to 5:1. The glass compositions may further contain up to 20 mole % of modifying oxides including up to 5 mole % SiO.sub.2, up to 20 mole % B.sub.2 O.sub.3, and up to 5 mole % Al.sub.2 O.sub.3. They may also contain one or more crystallization promoters selected from 1 to 5 mole % zircon and/or zirconia and 1-15 mole % R.sub.2 O. Additionally, the composition may include a seal adherence promoter selected from up to 5 mole % WO.sub.3, up to 5 mole % MoO.sub.3, up to 0.10 mole % Ag metal and mixtures.
Frits in this composition area exhibit good flow at sealing temperatures. With the addition of suitable expansion-modifying fillers, they can be made compatible with a wide range of substrates. Such mill additions are described in the Aitken et al. and Francis et al. patents mentioned earlier.
While these non-lead glass flits form good seals, they have presented difficulties with respect to surviving a bakeout and exhaust procedure. My related application, Ser. No. 08/512,618, filed Aug. 8, 1995 as a C-I-P of Ser. No. 08/221,400, addresses the problem with a sealing material consisting essentially of 60-90% of glass frit and 10-40% of a mill addition including 10-30% alumina and 0-30% zircon. While the mill addition is a necessary component of that invention, it is not consistently effective by itself. In addition, it is necessary to develop a small amount, preferably 5-10%, of a crystal phase in a seal produced with the material.
Development of sufficient crystal phase to be effective requires a time-temperature sealing cycle that is often impractical. Thus, the sealing temperature may range from a 450.degree. C. minimum up to 475.degree. C. The time required at the lower end of the temperature range may be an hour or more. Such a time-temperature cycle is, of course, well above the conventional cathode ray tube sealing cycle of 430-450.degree. C. for times of 30 to 45 minutes.
It is a purpose of the present invention to provide a fusion sealing material based on a SnO--ZnO--P.sub.2 O.sub.5 glass frit, and capable of producing a seal that remains rigid at a temperature as high as 550.degree. C. Another purpose is to provide a sealing material containing a SnO--ZnO--P.sub.2 O.sub.5 glass frit that can be substantially thermally crystallized. A further purpose is to provide a sealing material containing an additive that catalyzes such thermal crystallization. A still further purpose is to provide a sealing material that can be fused to a tight, rigid seal employing the conventional sealing cycle for cathode ray tubes.